The present invention relates to a light-modulation element, a GLV device, and a laser display. More particularly, the present invention relates to a light-modulation element including a combined light-reflective film and a membrane-side electrode having a high reflectance, a GLV device including the light-modulation elements, and a laser display including such a GLV device or devices.
Attendant on the progress of miniaturizing technology, attention has been paid to the so-called micromachine (MEMS: Micro Electro-Mechanical System) element and small-type apparatuses incorporating the MEMS element or elements.
The MEMS element is formed as a miniaturized structure on a substrate such as a silicon substrate a glass substrate, and the like, in which a driver for outputting a mechanical driving force and a semiconductor integrated circuit or the like for controlling the driving of the driver are coupled electrically and, further, mechanically. A basic characteristic of the MEMS element lies in that the driver constituted as a mechanical structure is incorporated in a part of the element, and the driving of the driver is performed electrically by application of a coulomb attractive force between electrodes or the like.
The constitution of a light-modulation element will be described by showing as an example the light-modulation element provided in the GLV (Grating Light Valve) device developed as a light-intensity conversion element, namely, a light modulator for laser display, by SLM (Silicon Light Machine) Company.
First, referring to FIG. 5, the structure of the GLV device including the light-modulation elements will be described. FIG. 5 is a perspective view showing the constitution of the GLV device.
As shown in FIG. 5, the GLV device 10 is a device in which a plurality of light-modulation elements 12 are disposed densely and in parallel to each other. Each of the light-modulation elements 12 constituting the GLV device 10 is a light-modulation element called MOEMS (Micro Optical Electric Mechanical System) including an electrostatic-driving-type membrane 16 having a light-reflective film 14 on the upper side thereof, and it has the function of modulating the intensity of light reflected by the light-reflective film 14 by diffraction of light through alternate variation of the height of the light-reflective film 14 as a result of mechanical movements of the membrane 16 by an electrostatic attracting force or an electrostatic repelling force.
Next, referring to FIG. 6, the constitution of the light-modulation element 12 will be described. FIG. 6 is a perspective view showing the constitution of the light-modulation element.
As shown in FIG. 6, the light-modulation element 12 includes an insulating substrate 18, such as a glass substrate, a substrate-side electrode 20 composed of a thin Cr film or the like and formed on the insulating substrate 18, and the electrostatic-driving-type membrane 16 crossing and being astride the substrate-side electrode 20 in a bridge form.
The electrostatic-driving-type membrane 16 and the substrate-side electrode 20 are electrically isolated from each other by a void portion 22 therebetween.
The electrostatic-driving-type membrane 16 includes a bridge member 24 composed of an SiN film provided as an electrode-support member and based on the substrate 18 bridgingly astride the substrate-side electrode 20, and a combined light-reflective film and membrane-side electrode 14 composed of an Al film of about 100 nm in thickness that is provided on the bridge member 24 oppositely to and in parallel to the substrate-side electrode 20.
The bridge member 24 is opposed to and spaced by a predetermined gap from the substrate-side electrode 20 so as to secure the void portion 22 therebetween, and it is provided for supporting the combined light-reflective film and membrane-side electrode 14 in parallel to the substrate-side electrode 20.
In the GLV device 10, the insulating substrate 18 and the substrate-side electrode 20 thereon are respectively a common substrate and a common electrode for the light-modulation elements 12, as shown in FIG. 5.
The electrostatic-driving-type membrane 16 constituted of the bridge member 24 and the combined light-reflective film and membrane-side electrode 14 provided thereon is a portion called a ribbon.
The bridge member 24 may in some cases be of the cantilever type in which only one end of a beam portion extending in parallel to the substrate-side electrode 20 is supported by one column portion, in place of the bridge form shown in FIG. 6 in which both ends of the beam portion are supported by two column portions, respectively.
The aluminum film (Al film) used as the combined light-reflective film and membrane-side electrode 14 is a metallic film preferable as an optical component material on the ground that (1) it is a metallic film which can be formed comparatively easily, (2) it has a small wavelength dispersion of light reflectance in the visible ray region, (3) a spontaneously oxidized alumina film formed on the surface of the Al film functions as a protective film for protecting the reflective surface, and the like.
On the other hand, the SiN film (silicon nitride film) constituting the bridge member 24 is a film deposited by a low-pressure CVD process. The SiN film is selected on the ground that its physical properties, such as strength and elastic constant, are suitable for mechanical driving of the bridge member 24.
When a minute voltage is impressed between the substrate-side electrode 20 and the combined light-reflective film and membrane-side electrode 14 opposed to the substrate-side electrode 20, the electrostatic-driving-type membrane 16 approaches the substrate-side electrode 20 due to an electrostatic phenomenon, and when the impressing of the voltage is stopped, the electrostatic-driving-type membrane 16 is spaced away from the substrate-side electrode 20 into its original state.
Each of the light-modulation elements 12 constituting the GLV device 10 modulates the intensity of the light reflected by the light reflective film 14 by diffraction of light as a result of alternative variation of the height of the light-reflective film 14 through the approaching and spacing actions of the electrostatic-driving-type membrane 16 relative to the substrate-side electrode 20.
The dynamic characteristics of the membrane 16 driven by utilizing the electrostatic attracting force and electrostatic repelling force are substantially determined by the physical properties of the SiN film formed by a CVD process or the like, and the Al film plays a main role as a mirror or reflector.
In the conventional light-modulation element, however, since the combined light-reflective film and membrane-side electrode is composed of an Al film, there have been the following problems. Namely, although the Al film has the above-mentioned merits, the material is comparatively low in melting point and is soft. Therefore, when a thin Al film of about 100 nm in thickness is formed, differences in thickness of the Al film are generated due to aggregation of aluminum. In addition, when the differences in height are as large as not less than 400 nm, a rough surface reaching to 1 xcexcm is generated in the Al film, and light reflectance of the Al film is lowered. For example, while the bulk Al film has a light reflectance of about 92% for light with a wavelength of 600 nm, the light reflectance of the conventional light-modulation element 10 is about 86.5% for the wavelength of 600 nm; thus; there is a lowering in reflectance by about 5%.
Besides, when the differences in thickness generated in the Al film are extremely large, the Al film may be broken, resulting in an electrically non-conductive state. In such a case, driving of the membrane cannot be achieved, and the light-modulation element cannot function.
Accordingly, it is an object of the present invention to provide a light-modulation element including a combined light-reflective film and membrane-side electrode being uniform in thickness and having a high light reflectance.
The present inventors found out that the variation in the thickness of the Al film is generated as a result of the promotion of migration of aluminum because the Al film formed on an amorphous SiN film is formed by a low-pressure CVD process.
However, it is difficult to find an appropriate membrane film that can be used in place of the SiN film, and it is preferable to utilize the good light reflectivity of Al film by providing the Al film as an upper layer, from the viewpoint of light reflectivity.
In view of this, the present inventors gained the idea of intermediately providing a high-melting-point metal film, for example, a titanium film, and forming crystals with a columnar, fine-grain structure between the SiN film and the Al film so as to restrain the migration of aluminum by the good surface condition of the high-melting-point metal film. The present inventors have confirmed the effects of this idea by experiments to reach to the present invention.
In accordance with one aspect of the present invention, there is provided a light-modulation element including a substrate-side electrode provided on an insulating substrate, and a membrane including a bridge member extending above and spaced from the substrate-side electrode in the manner of crossing the substrate-side electrode, the bridge member having at least an upper layer thereof composed of an SiN film, and a combined light-reflective film and membrane-side electrode composed of a metallic film provided on the SiN film of the bridge member oppositely to the substrate-side electrode, the membrane driving said light-reflective film by an electrostatic attracting force or an electrostatic repelling force acting between the combined light-reflective film and membrane-side electrode and the substrate-side electrode, wherein a multilayer metallic film including an Al film constituted mainly of aluminum (Al) and at least any of a high-melting-point metal film, a high-melting-point metal nitride film, and a high-melting-point metal carbide film provided under the Al film is provided as the combined light-reflective film and membrane-side electrode.
The present invention can be applied to a light-modulation element in which at least an upper layer of a bridge member is composed of an SiN film. The bridge member may be entirely composed of the SiN film, may have a two-layer structure constituting an upper layer composed of the SiN film and a lower layer composed of a film other than the SiN film, for example, an SiO2 film, and, further, may have a three-layer structure.
The Al film may be an Alxe2x80x94Si alloy film, an Alxe2x80x94Cu alloy film, an Alxe2x80x94Cuxe2x80x94Si alloy film, or an Al film containing other impurities.
In the present invention, the Al film is formed on any of a high-melting-point metal film, a high-melting-point metal-nitride film, and a high-melting-point metal-carbide film which has good crystallinity with a columnar, crystal structure so that the Al film has a good crystal structure, and aggregation phenomenon would not easily occur.
Therefore, no variation is generated in the thickness of the Al film, so that the Al film has a smooth reflective surface, light reflectance is enhanced, and light utilization efficiency of the light-modulation element is enhanced.
Preferably, the multilayer metallic film provided as the combined light-reflective film and membrane-side electrode includes a high-melting-point metal film provided on the SiN film of the bridge member and a high-melting-point, metal-nitride film or a high-melting-point, metal-carbide film provided on the high-melting-point metal film.
The high-melting-point metal film has the effect of enhancing adhesion between the high-melting-point, metal-nitride film or the high-melting-point, metal-carbide film and the SiN film.
More preferably, the high-melting-point metal film is any of a titanium (Ti) film, a tungsten (W) film, a molybdenum (Mo) film, and or tantalum (Ta) film, and the high-melting-point, metal-nitride film or the high-melting-point, metal-carbide film is a nitride film or a carbide film of the any of the titanium (Ti) film, the tungsten (W) film, the molybdenum (Mo) film, or the tantalum (Ta) film.
In accordance with another aspect of the present invention, there is provided a GLV device including a plurality of light-modulation elements disposed in parallel to each other, wherein each of the light-modulation elements includes a substrate-side electrode provided on an insulating substrate, and a membrane including a bridge member extending above and spaced from the substrate-side electrode in the manner of crossing the substrate-side electrode, the bridge member having at least an upper layer thereof composed of an SiN film, and a combined light-reflective film and membrane-side electrode composed of a metallic film provided on the SiN film of the bridge member oppositely to the substrate-side electrode, the membrane driving the light-reflective film by an electrostatic attracting force or an electrostatic repelling force acting between the combined light-reflective film and membrane-side electrode and the substrate-side electrode, wherein the combined light-reflective film and membrane-side electrode is a multilayer metallic film including an Al film constituted mainly of aluminum (Al) and at least any of a high-melting-point metal film, a high-melting-point, metal-nitride film, and a high-melting-point, metal-carbide film provided under the Al film, the combined light-reflective films and membrane-side electrodes are independent from each other and disposed in parallel to each other, and the substrate-side electrode is provided as a common electrode.
In accordance with a further aspect of the present invention, there is provided a laser display including a laser and a GLV device disposed on the optical axis of laser light emitted from the laser to modulate the intensity of the laser light, wherein the GLV device includes a plurality of light-modulation elements disposed in parallel to each other; each of the light-modulation elements includes a substrate-side electrode provided on an insulating substrate, and a membrane including a bridge member extending above and spaced from the substrate-side electrode in the manner of crossing the substrate-side electrode, the bridge member having at least an upper layer thereof composed of an SiN film, and a combined light-reflective film and membrane-side electrode composed of a metallic film provided on the SiN film of the bridge member oppositely to the substrate-side electrode, the membrane driving the light-reflective film by an electrostatic attracting force or an electrostatic repelling force acting between the combined light-reflective film and membrane-side electrode and the substrate-side electrode, wherein the combined light-reflective film and membrane-side electrode is a multilayer metallic film including an Al film constituted mainly of aluminum (Al), and at least any of a high-melting-point metal film, a high-melting-point, metal-nitride film, and a high-melting-point, metal-carbide film provided under the Al film, the combined light-reflective films and membrane-side electrodes are independent from each other and disposed in parallel to each other, and the substrate-side electrode is provided as a common electrode.
In the laser display according to the present invention, there is no limitation as to the number of lasers; the laser display may be a monochromic laser display or a full-color display.
In the case of the full-color display, the laser display includes: a red laser, a green laser; a blue laser; a color-combination filter for combining red laser light, green laser light and blue laser light emitted respectively from the red laser, the green laser and the blue laser; and GLV devices disposed on optical axes between the red laser, the green laser, the blue laser, and the color-combination filter for modulating the intensities of the red laser light, the green laser light and the blue laser light emitted respectively from the red laser, the green laser and the blue laser.
According to the present invention, since the ground for the Al film is the high-melting-point metal film or the like, degradation of surface morphology due to aggregation of aluminum in the Al film can be prevented, and a light-modulation element having a high reflectance and, hence, a high light-utilization efficiency can be realized.
By this, it is possible to enhance the reliability of the light modulation element and to increase the production process margin of the light-modulation element.
Further, by composing the GLV device of the light-modulation elements according to the present invention, it is possible to realize a GLV device having a high light-utilization efficiency and a long useful life. Besides, by incorporating such a GLV device or GLV devices in a laser display, it is possible to realize a laser display having a high light-utilization efficiency.